Introduction

• Body temperature is an important parameter routinely measured as part of a clinical examination.  
• Pyrexia (fever) is defined as an increase in core body temperature.  
• In the adult cat the normal temperature range is 37.7-39.1°C Normal vital signs. 
• In the neonatal kitten it is slightly lower (33.3-36.1°C) increasing to 37.7°C by 4 weeks.

Temperature homeostasis

• The thermoregulatory center in the hypothalamus is responsible for regulation of core body temperature, ensuring the core temperature is tightly controlled within a narrow range of the set point. This is based on information received from peripheral and central thermoreceptors and allows normal body homeostasis to occur.  
• The central temperature receptors are found in the anterior hypothalamic-preoptic area. The anterior hypothalamic-preoptic area and the peripheral receptors transmit signals into the posterior hypothalamic area, which activates autonomic and behavioral responses to regulate body temperature.  
• The body temperature may be elevated due either to pyrexia or hyperthermia.  
• In cases of true pyrexia the set-point is reset to a higher level and this is recorded as a higher rectal temperature.  
• In hyperthermia, the set point of the hypothalamus has not been altered. Increased body temperature occurs due to either excessive heat production, eg intense exercise/seizure activity; or ineffective heat dissipation, eg heat stroke Hyperthermia.
• Some pathological conditions, such as malignant hyperthermia and central nervous system disorders, can also induce hyperthermia. Hyperthermic conditions do not respond well to antipyretic medication. Often, the underlying cause will be evident from the history, helping to distinguish between pyrexia and hyperthermia.

Mechanisms of pyrexia

• True pyrexia is induced by the production of endogenous pyrogens or cytokines, eg IL-1α, IL-1β IL-6, TNFα, from activated immune cells, especially macrophages and monocytes, in response to exogenous pyrogens in the body.  
• Exogenous pyrogens are numerous and include the products and components of infectious agents (bacteria, viruses, fungi, protozoa), bacterial products (eg enterotoxins, lipopolysaccharides, etc), immune complexes, tissue inflammation or necrosis, and several pharmacologic agens. Some neoplastic cells may also produce cytokines that result in a febrile response.
• The endogenous pyrogens are responsible for resetting the thermoregulatory centers set point by causing prostaglandin release. This is why COX inhibitors are effective in reducing fevers through their inhibitory effects on the arachidonic acid pathway.

Systemic effects of pyrexia

• As a physiological response, elevations in body temperature can have both advantageous and adverse effects.
• Improved host defenses in pyrexic animals with bacterial infections have been associated with better survival rates, reduced duration of disease. Although this is controversial.  
• Fever is commonly considered to kill bacteria, but this is likely untrue and bacteria continue to replicate at temperatures that would otherwise prove fatal to animals and people.
• However once significant elevations in temperature occur for prolonged periods, detrimental effects are evident, including increased number and severity of clinical signs, increased metabolic rate, increased oxygen demand and muscle wastage and weakness.

History and clinical examination

Pyrexia of unknown origin (PUO)

Diagnostics

Treatment

Special risks

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Spencer S E, Knowles T, Ramsey I K et al (2017) Pyrexia in cats: Retrospective analysis of signalment, clinical investigations, diagnosis and influence of prior treatment in 106 referred cases. J Feline Med Surg 19 (11), 1123-1130 PubMed.
• Flood J (2009) The diagnostic approach to fever of unknown origin in cats. Comp Contin Educ Vet 31 (1), 26-31 PubMed.
• Dunn J K & Gorman N T (1987) Fever of unknown origin in dogs and cats. JSAP 28 (3), 167-181 VetMedResource.

Other sources of information

• Ramsey I K, Tasker S (2017) Fever. In: Textbook of Veterinary Internal Medicine. 8th edn, Chapter 48.
• Lappin M (2013) Pyrexia and Hyperthermia. In: Harvey A M & Tasker S (eds) BSAVA Manual of Feline Practice: A Foundation Manual. Gloucester: British Small Animal Veterinary Association. pp 287-289.
• Lunn K F (2012) Fever. In: Greene C E (ed) Infectious Diseases of the Dog and Cat. 4th edn. USA: Elsevier. pp 1115-1123.